I think the plastic version was Argosy only and I am certain they are no longer available .
If you want something less expensive you can use this type . It actually is a better vent cover and does not really look out of place once painted to match the trailer .http://www.vintagetrailersupply.com/...-p/vts-517.htm

That's one fine renovation, only dream my 76 with queen middle fold out gaucho will be any thing close to that beautiful job. Yep thought your response is the answer. But if going to paint would plastic screw down type work equal well?

The plastic would be ok short term. If painted it would certainly help with the UV protection. Again, they are plastic so the base eventually would split where the screw secures it to the trailer. If you have an RV supplier near by they might stock metal covers . Thanks for the compliments !